1. Field of the Invention
The invention relates to an electric motor control apparatus that controls an electric motor system including, for example, a three-phase alternating-current motor.
2. Description of Related Art
There is pulse width modulation (PWM) control as an example of a control method to drive a three-phase alternating-current motor. In PWM control, a power converter that converts a direct-current voltage (direct-current power) to alternating-current voltages (alternating-current powers) is controlled on the basis of a magnitude relation between each of phase voltage command signals and a carrier signal having a predetermined frequency (see Japanese Patent Application Publication No. 2004-120853 (JP 2004-120853 A)). Each of the phase voltage command signals is set in terms of bringing phase currents that are supplied to the three-phase alternating-current motor into coincidence with desired values. PWM control may also be used to control a power converter that converts an alternating-current voltage to a direct-current voltage (see Japanese Patent Application Publication No. 2010-263775 (JP 2010-263775 A)).
Incidentally, a smoothing capacitor for suppressing fluctuations in direct-current voltage that is input to a power converter or that is output from the power converter is often electrically connected in parallel with the power converter. In recent years, the size of the smoothing capacitor is reduced by reducing the capacitance of the smoothing capacitor in many cases. However, if the capacitance of the smoothing capacitor is reduced, there is a concern that a ripple (so-called pulsation component) in the terminal voltage of the smoothing capacitor relatively increases. Therefore, in order to suppress (reduce) such a ripple in the terminal voltage of the smoothing capacitor, a technique for utilizing a triple harmonic signal is described in JP 2010-263775 A and JP 2004-120853 A. Specifically, JP 2010-263775 A describes a technique for controlling a switching element of the power converter such that the current waveform of an input current from an alternating-current power supply coincides with an associated wave of a sinusoidal wave having the same frequency as that of the alternating-current power supply with a triple harmonic wave of the sinusoidal wave. JP 2004-120853 A describes a technique for controlling an inverter circuit, which is an example of a power converter, by executing PWM control that uses modulated waves respectively obtained by superimposing corresponding triple harmonic waves on three-phase modulated waves.
A power converter generally converts a direct-current voltage to an alternating-current voltage by switching the states of switching elements of the power converter from an on state to an off state or from the off state to the on state. In such a power converter, there arises a loss due to switching (that is, switching operation) of the state of each of the switching elements. Thus, in the viewpoint of effectively utilizing a power converter, it is desirable to reduce the loss due to the switching operation of each switching element.
The loss due to the switching operation of each switching element increases as the number of switching operations of each switching element per unit time increases. Thus, it is possible to reduce the loss due to the switching operation of each switching element by reducing the number of switching operations of each switching element per unit time.
However, in the techniques described in JP 2010-263775 A and JP 2004-120853 A, there is a technical inconvenience that it is not possible to sufficiently reduce the number of switching operations of each switching element because a principal object is to suppress a ripple in the terminal voltage of the smoothing capacitor. Thus, there is room for improvement in terms of effectively utilizing a power converter in the techniques described in JP 2010-263775 A and JP 2004-120853 A.